The present invention relates to an Li-Mn type ferrite carrier for use in a two-component developer for developing electrostatic latent images formed on a surface of an image-bearing member (photosensitive drum) of image-forming apparatus such as laser beam printers, dry copying machines, etc. More specifically, the present invention relates to an Li-Mn type ferrite carrier having a high efficiency to triboelectrify the toner, enhancing the bonding strength between a resin coating and the surface of the carrier, and having a long useful life.
In a known electrophotographic developing process utilized in printers, facsimile machines, etc., an electrostatic latent image corresponding to original image or information data being reproduced is formed on the surface of a cylindrical photosensitive drum. A developing roll composed of a sleeve and a permanent magnet mounted interiorly in the sleeve is disposed opposite to the photosensitive drum. A magnetic developer is magnetically attracted on the surface of the sleeve and transported to a developing zone by the rotation of the sleeve. The magnetic developer forms a magnetic brush in the developing zone, and the surface of the photosensitive drum is brushed by the magnetic brush to develop the electrostatic latent image to a visual toner image. The toner image is transferred onto a recording sheet which is then heated to permanently fix the toner image thereon.
As the developer, one-component developer containing only a toner or two-component developer which is a mixture of a toner and a magnetic carrier has been used. In the two-component developer development, the toner and the magnetic carrier are mixed together in a predetermined mixing ratio, and acquire triboelectric charges of opposite polarities by friction. During the brushing contact, only the toner charged to a predetermined polarity is deposited on the image areas on the photoconductive surface by electrostatic attraction between the latent image and the toner to produce visual toner images.
Generally, the two-component developer is required to have a good stability to reproduce images with a high quality in continuous developing operation. To attain a good stability, it is necessary to regulate the ability of the magnetic carrier to triboelectrify the toner and the specific volume resistance of the developer within a suitable level. In addition, it is desirable that these properties are not changed even in a long developing operation.
The toner is a fine powder usually produced by blending a binder resin with several additives such as a colorant including dyestuff and pigment, a magnetic powder, a charge control agent, a wax, etc. to disperse the additives through the binder resin, and pulverizing the blended product to a predetermined particle size.
As the material for the carrier, iron powder and a ferrite powder have been mainly used. To make the ability to triboelectrify the toner and the specific volume resistance stable, several considerations have been made. Such considerations include, for example, developing new binder resin, charge control agent, or other additives for the toner, selecting the binder resins, charge control agents, or other additives for the toner, improving the method of oxidation-treating the iron carriers, selecting the material for the ferrite carriers and the surface resin coating, studying the surface property of the carriers, improving the method of resin coating, etc.
The iron carrier is usually produced by subjecting to its surface a suitable treatment such as oxidation treatment. However, in spite of such a surface treatment, the iron carrier physically or chemically changes its surface conditions in a longtime use to produce spent toners (toners fused to carriers). The spent toners reduce the ability of the carrier to triboelectrify the toner. The iron carrier has additional defects that the resolution of the images comes to be reduced because the iron carrier is sensitive to moisture in the environmental atmosphere, and that the lift time is short.
The ferrite carrier has come to be used in place of the iron carrier because it is chemically more stable than the iron carrier, it hardly changes its specific volume resistance during the use and its apparent density is about 2/3 of that of the iron carrier. The ferrite carrier is a sintered product of a complete mixture of an iron (III) oxide and at least one oxide of a metal such as Ni, Zn, Mn, Mg, Cu, Li, Ba, V, Cr, Ca, etc., and the ferrite carrier generally used may be made of Ni--Zn ferrites, Mn--Zn ferrites and Cu--Zn ferrites. However, Zn, Ni, etc. are noxious to human body and the disposal of waste ferrite carrier containing such elements is legislatively regulated. Li--Mn ferrites are not legislatively regulated, and therefore, such an advantage of the Li--Mn ferrite carrier is recently noted in the art. However, since the optimal chemical composition and the optimal developing conditions have not yet been established, it is difficult at present to stably reproduce images with a high quality by the use of a developer containing the Li--Mn carrier in a continuous developing operation.
To increase the property of the carrier to triboelectrically charge the toner and prevent the surface coating from exfoliating from the carrier core, it has been proposed to make the carrier surface rough.
Japanese Patent Publication No. 8-1525 proposes a method of forming a rough surface on ferrite carriers due to the geometric shape of primary particles. In the proposed method, a ferrite powder having a small primary particle size of 0.5 to 7 .mu.m is formed into spherical granulates by spray-drying, and then the granulates are sintered. Japanese Patent Publication No. 8-1525 teaches that the sintering temperature be selected so that the primary particles on the surface of the granulates retain their geometric shape, i.e., so that the primary particles on the surface of the granulates are not fused to form a continuous phase. To ensure the formation of the surface roughness, the sintering temperature is taught to be 900 to 1100.degree. C.
Japanese Patent Publication No. 8-12489 also teaches a ferrite carrier having a surface roughness due to the fine primary crystalline particles. It is taught therein that the sintering temperature is an important factor to form the rough surface because the primary particles are fused to form a smooth surface at a high sintering temperature. Therefore, Japanese Patent Publication No. 8-12489 teach that the sintering be carried out at a relatively low temperature although the sintering temperature generally employed is 1000 to 1400.degree. C.
However, the ferrite particles are insufficiently sintered by the above methods because the sintering temperature is low to retain the geometric shape of the primary particles, and therefore, the ferrite particles are not sufficiently densified. As a result thereof, the carrier thus obtained has a low magnetic force to result in a frequent carrier adhesion to the photosensitive surface. This causes a problem of a poor image quality due to occurrence of white spots in the reproduced image area. Since the surface roughness due to the geometric shape of the primary particles is large and deep, the carrier is not well-agitated with the toner due to its low flowability, thereby reducing the property of the carrier to triboelectrically charge the toner. Therefore, ample time is required to charge the toner to a sufficient level.